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May 23, 2012 -- Aspirin may be effective long-term to reduce the risk of repeat blood clots in veins, new research suggests.

Patients who have these potentially dangerous blood clots, called venous thromboembolism or VTE, often get them in deep veins in the legs. They can then travel up to the lungs and are sometimes fatal.

When the first one occurs, a patient is typically put on an anti-clotting drug, or anticoagulant, for several months. However, staying on these drugs is linked with an increased risk of dangerous major bleeding.

Now, researchers have found that daily aspirin may be a good next step after completing treatment with the initial anti-clotting drug.

"Based on our results, we believe that aspirin can be considered an alterative to extended oral anticoagulant treatment for secondary prevention [of blood clots]," says researcher Cecilia Becattini, MD, PhD, of the University of Perugia in Italy.

Those on aspirin, she found, were about 40% less likely than those on placebo to have a repeat blood clot.

A U.S. expert calls the study findings exciting but says more study is needed.

Blood Clots: The Problem

Doctors know that the risk of a repeat blood clot endures for many years. So they prescribe the anti-clotting drugs for several months after the initial blood clot.

About 20% of patients who have a blood clot without any known risk factors, such as having recent surgery, will have another one within two years after the anti-clotting drugs are stopped.

The drugs are stopped to decrease the risk of major bleeding. "Major bleeding means intracranial, life-threatening, or even fatal bleeding," Becattini tells WebMD.

When patients are on the anti-clotting drugs, such as warfarin (Coumadin, Jantoven), they also have to get blood tests often to see if the dose is OK.

Blood Clots: The Study

Becattini's team evaluated 402 men and women. They had been on anti-clotting drugs for six to 18 months after having a blood clot.

The researchers assigned about half of the study participants to take 100 milligrams of aspirin a day for two years and half to take a placebo pill every day for two years.

While 28 of the 205 patients on aspirin, or 6.6% per year, had another blood clot, 43 of the 197 on placebo, or 11.2% per year, did.

Aspirin reduced the risk of repeat clots while not increasing the risk of major bleeding, Becattini found. Aspirin is among a class of drugs that does have a risk for stomach bleeding with long-term use.

In each group, one patient had a major bleeding problem. Other side effects, such as gastric pain, were similar in the two groups

Blood Clots: Perspective

The study findings are ''exciting," says Richard C. Becker, MD, professor of medicine at the Duke University School of Medicine. He wrote an editorial to accompany the study.

However, he tells WebMD, "patients should not be doing things differently than they are now."

"The most important message for the readers is that additional studies need to be done," he says.

The new study adds to growing evidence that aspirin may help, he says. But he says experts should wait until the results of yet another study are in, expected later this year.

The other study is known as ASPIRE (Aspirin to Prevent Recurrent Venous Thromboembolism). Researchers have already enrolled 822 patients, he says.

It, too, is comparing 100 milligrams of aspirin vs. placebo a day. The patients had already been on anti-clotting drugs for three to six months.

Researchers have already planned to combine the results of the two studies to look more closely at the risks and benefits, Becker tells WebMD.

Becattini presented some results of her study late last year at the annual meeting of the American Society of Hematology.

Partial funding for the study came from a grant-in-aid from Bayer HealthCare.

SOURCES:Becattini, C. The New England Journal of Medicine, May 24, 2012.Becker, R. The New England Journal of Medicine, May 24, 2012.Cecilia Becattini, MD, PhD, researcher, internal medicine, University of Perugia, Italy.Richard C. Becker, MD, professor of medicine, Duke University School of Medicine, Durham, N.C.